1. Field of the Invention
This invention relates to generation of radio frequency signals and more specifically to a multiple phase locked loop frequency synthesizer.
2. Description of the Prior Art
Generally any electronic device operating at radio frequencies requires generation of local oscillator (LO) high frequency signals. This is true of radio transmitters and receivers, for instance of the type included in cellular telephones, cordless telephones, television sets, etc. Local oscillators typically used in wireless communications require a very high frequency; e.g., a digital cordless telephone requires a 900 MHz oscillator frequency.
Implementation of these high frequencies in the prior art requires relatively expensive circuitry, an example of which is the National Semiconductor Corp. LMX9301 hybrid frequency synthesizer. This device uses co-fired ceramic external components for the VCO (voltage controlled oscillator) and loop filter. A block diagram of this device including the required external components is shown in FIG. 1. This device has only one PLL (phase locked loop) so that the phase noise of the loop is dominated by the size of the feedback divider which is directly related to the VCO output frequency channel spacing. While this device achieves adequate channel spacing, it generates unwanted reference spurs at the channel spacing frequency. In addition, use of this device requires the external VCO as shown in FIG. 1 which contains a non-integrable high Q inductor. (Non-integrable means this cannot be located on an integrated circuit with the other elements of the circuit). Single chip synthesizers which do not require external components do not provide adequate channel spacing and also require external component filters.
The FIG. 1 circuit has an input terminal 10 to which is applied an oscillating frequency f.sub.OSC which is coupled to a divide by R digital element 14 which in turn drives one terminal of a phase comparator 16. The output terminal of the phase comparator 16 drives a charge pump 20. The external components include the loop filter 26 and the VCO 30 which require the above-mentioned low temperature co-fired ceramic fabrication technology. The output signal of the VCO 30 in turn drives a divide by N element 32 which is coupled to the second input terminal of phase comparator 16. The desired output signal frequency is the output signal frequency of the VCO 30.
Hence it would be desirable to have a circuit for generating a high frequency which does not require the external components of FIG. 1 and also generates less phase noise.